Manholes and similar apertures of the type here considered are generally provided in the walls of containers whose interior must be accessed from time to time. Normally, a cover--which in some instances must be able to withstand internal pressures--is secured to the aperture by bolts or other suitable fasteners so that a tight seal is formed between confronting annular contact surfaces of the cover and the aperture rim. After every removal of the cover these contact surfaces must be carefully cleaned to remove incrustations and other adhering solids which would interfere with the subsequent reclosure. The cleaning of manhole rims has heretofore generally been done manually, e.g. with the aid of simple wire brushes or of hand-held tools with mechanically driven scrubbers. Such manual cleaning is cumbersome and potentially dangerous, especially in the case of containers serving to store radioactive materials in a nuclear-power plant; even persons in protective suits must not spend extended periods in such an environment so that the work must be performed in a hurry. Moreover, a uniform cleansing of the aperture rim is difficult to achieve by hand even under more favorable circumstances.